Electrodes for electrolytic cells

ABSTRACT

An electrode for use in an electrolytic cell, and a method for producing same, wherein said electrode comprises an internal copper conductor and an external element of a second metal, at least a portion of each having contact surfaces being held in intimate contact with the other, said conductor and said element each having a conductive coating applied to the contact surface, said conductive coating comprising between about 20 and about 30 percent indium and between about 80 and about 70 percent gallium, whereby the contact resistance between said conductor and said element is reduced.

.Iadd.This application is a reissue of U.S. Pat. No. 4,452,685.

FIELD OF THE INVENTION

This invention relates to an improved electrode for use in chlor-alkalielectrolytic cells and a method for producing same.

PRIOR ART STATEMENT

In view of the phenominal jump in energy prices and increased scarcityof industrial fuel supplies, there has been a continuous activity in theelectrolysis field to found ways to reduce the amount of power used inelectrolytic processes. In the chlor-alkali industry, such activity hasbeen concerned with the development of dimensionally stable anodes,catalytic cathodes and advanced membrane cell structures, all of which,when combined, have resulted in significant decreases in the amount ofenergy required for per ton of product. In such cells, it is mostimportant that the current density within the volumetric space betweenthe anodes and the cathodes be as uniform as possible. This bothminimizes wear and tear on the membrane and intends to maximize theproduction rate within the cell, all other conditions being equal. Sucha condition is achieved by using anodic and cathodic structures whichare adapted to uniformly distribute power across the surface area of theelectrode. This is generally accomplished by building into the anode andcathode structures at least one, but more usually a plurality of centralinternal conductors usually of copper which are adapted to act asextensions of the associated bus bar power distribution system andpromote even distribution of electric current throughout the externalportions of the electrode structure.

In the design and operation of such a system, it is found that onefactor which tends to limit the absolute quantity of current which canbe so distributed is the contact resistance between the centralconductors and the external electrode structure. The materials used forsuch a structure, normally titanium for anodes and nickel for cathodes,have substantially different electric conductivities as compared tocopper. Further, in the case of titanium and nickel, there is a strongtendency to build up a thin oxide layer on the exposed surfaces, saidlayer being relatively nonconductive and resulting in rather significantcontact resistance values between a central conductor and the externalelectrode structure. In recognition of this, a number of attempts havebeen made to produce electrode structures having lower contactresistance. These include such techniques as plating or coextruding anexternal coating of nickel or titanium onto the central copper conductorand then welding or bonding in some way the rest of the structure ontothis external coating. In other designs, a thin film of copper may besputtered onto the mating surfaces of the nickel and titanium electrodecomponents to establish a nominally low resistance Cu--Cu couple at thisinterface. In still other cases, the components are tightly clampedtogether so that physical pressure tends to cause the oxide layer tobreak up, thus reducing the effective contact resistance between them.Such techniques have proven to be reasonably effective but can beexpensive to implement. Further, with the ever increasing emphasistowards higher power levels in the cell, it becomes more and moredifficult to effectuate them in a way which does not eventually causeother problems in cell operation.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an electrode having a lowerelectrical contact resistance between its components.

It is a further object of this invention to provide a method forreducing the contact resistance between copper and titanium or nickel.

These and other objects of this invention will become apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE INVENTION

The objects of the present invention are met by providing an electrodefor use in an electrolytic cell, said electrode being comprised of aninternal central copper conductor and an external element, said elementbeing selected from the group consisting of titanium and nickel, atleast a portion of each having contact surfaces which are held inintimate contact with the other, said conductor and said element havinga conductive coating between said contact surfaces, said conductivecoating comprising between about 20 to about 30 percent indium and about80 and about 70 percent gallium, whereby the contact resistance betweensaid conductor and said element is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an exemplary electrode assembly as used in achlor-alkali cell.

FIG. 2 is a cross section of the central conductor of FIG. 1 along theline 2--2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can best be understood by reference to anexemplary structure to which it is applied. This is an electrode of atype described for use in a chlor-alkali cell as shown in U.S. Pat. No.4,222,831 issued to Specht et al. on Sept. 16, 1980, which isincorporated by reference to the extent relevant hereto. As shown inFIGS. 1 and 2, electrode 10 comprises basically an envelope having afront electrode surface 12 and a back electrode surface 14, saidelectrode surfaces being either of a solid, mesh, expanded metal orforaminous nature.

Emplaced within the interior of electrode 10 is at least one, but moreusually a plurality of central power distributors 16 which comprise aninternal conductor 18, preferably of copper, extending substantiallyacross the width of electrode 10. In the embodiment illustrated, this isthreadingly adapted at its outermost end 20 to engage a bus bar or cablefrom an external power system (not shown).

In the preferred embodiment for this invention, the external portions ofsaid front and back electrode surfaces 12 and 14 for anodic use are madefrom titanium and for cathodic use are made from nickel. Typicalmaterials utilized for internal conduction 18, anode surfaces andcathode surfaces, respectively, are copper C 110, nickel 200 andcommercial titanium (Grade 1). Nominal compositions quoted for thesematerials are:

    ______________________________________                                        Copper (C 110) Cu          99.9%   (min.)                                                    O           0.005%  (max.)                                     Nickel (200)   Ni + Co     99.0%   (min.)                                                    C           0.15%   (max.)                                                    Cu          0.25%   (max.)                                                    Fe          0.40%   (max.)                                                    Mn          0.35%   (max.)                                                    Si          0.35%   (max.)                                                    S           0.01%   (max.)                                     Titanium (Gr. 1)                                                                             Ti          99.6%   (min.)                                                    N           0.3%    (max.)                                                    C           0.10%   (max.)                                                    H           0.015%  (max.)                                                    Fe          0.2%    (max.)                                                    O           0.18%   (max.)                                     ______________________________________                                    

However, other similar metals may be used if desired.

As shown in FIG. 2, internal conductor 18 is surrounded by an externalelement 22 which is a concentric, intimate physical sheath in contactwith internal conductor 18. External element 22 is also in more or lesscontinuous electrical contact with front and back surfaces 12 and 14.

Both nickel and titanium are known to form tightly adherent surfaceoxide layers which will act to electrically insulate the interiorcontact surface of element 22 from the mating exterior surface ofinternal conductor 18. This acts to raise the contact resistance betweenthe two, thus increasing the voltage drop across interstitial area 24.

In the process of this invention, such contact resistance losses betweeninternal conductor 18 and external element 22 are substantially reducedby coating the mating surfaces of said conductor 18 and said element 22with a thin layer of a conductive coating 25, said coating acting tofill interstitial area 24. Conductive coating 25 is a liquid metalmixture comprised of between about 20 and about 30 percent indium byweight and between about 80 and about 70 percent gallium by weight, andpreferably between about 23 and about 26 percent indium and betweenabout 77 and about 74 percent gallium. Conductive coating 25 is a highlyfluid eutectic having a melting point of approximately 18° C. so that itcan be easily applied to said surfaces even at room temperature.Further, unlike mercury or other low melting alloys, conductive coating25 does not immediately amalgamate or otherwise act to bond conductor 18and external element 22 together.

Conductive coating 25 is preferably applied to relatively clean surfacesand may be wiped on with a suitable applicator such as a paint brush,cotton swab or wiping cloth. For larger areas, squeegees or suitablydesigned spray equipment may also be used. Where the surfaces arecontaminated, some degree of precleaning is required to promote goodwetting. For light dirt, this may comprise operations such as degreasingor washing with strong detergents. For more heavily contaminatedsurfaces, particularly heavily oxidized surfaces, acid etching or alight dressing with an abrasive-containing material, such as an abrasiveimpregnated foam or fine sandpaper having a grit size between about 80and about 400, may also be utilized. Further, it is found that where theabrasive containing material itself is either impregnated or coated withsaid conductive coating, oxide removal and application can be conductedmore or less simultaneously. All of these operations can be conducted atroom temperature due to the low melting point of the eutecticcomposition. The surfaces should be dry prior to said application, andany debris or excess coating material remaining after the surfaces havebeen evenly coated, removed.

To minimize galvanic corrosion problems resulting from contact with theanolyte or catholyte solutions, external element 22 is preferably madefrom the same material as that used for said surfaces, i.e. titanium foranodic use and nickel for cathodic use. Further, to both maximizecurrent transfer and promote structural rigidity, external element 22 isusually welded to said front and back surfaces.

Utilizing conductive coating 25 in accordance with the process of thisinvention permits the use of less costly procedures to assemble thebasic electrode. Thus, for assembly of electrode 10 shown in FIGS. 1 and2, the outer electrode surfaces 12 and 14 and external element 22 can beprefabricated without the necessity of having a built-in internalconductor 18. When such an item is needed to complete the assembly, itmerely requires that the mating surfaces be coated with conductivecoating 25 and the internal conductor 18 then inserted into the interiorof exterior element 22 to complete the overall assembly operation. Withproper tolerances, interstitial area 24 is completely filled withconductive coating 25 and good electrical contact is established withoutthe necessity of initial, permanent physical bonding between the twostructures.

It is not known exactly how conductive coating 25 works but it ispostulated that it works by filling interstitial area 24 with conductivematerial. Where a light oxide is present, it appears to either dissolveor displace said oxide, thus preventing recontamination of the cleanedsurface. When the compound is used to fill interstitial area 24 betweensurfaces of copper and nickel, it is found that the total resistance ofa copper-nickel couple is reduced from between about 0.5 and about 0.6milliohms to between about 0.07 and about 0.21 milliohms. Further, suchvalues do not seem to change much even after long-term contact at atemperature of about 90° C., whereas uncoated couples change rapidly anddrastically for the worse in times as short as .[.for.]. .Iadd.four.Iaddend.days or even less.

EXAMPLE 1

Two coupons of copper C 110 strip, each being 0.045"×1"×2" were cleanedby degreasing with methanol and acid etching in a 12 weight percent H₂SO₄ solution for about 10 seconds to produce a material having a frontto back resistance of about 0.06 milliohms. At the same time, twocoupons of nickel 200 alloy, each being 0.055"×1"2" were cleaned byvapor degreasing in methanol and acid etching in a solution comprising37.8 milliliters H₂ O+56.8 milliliters H₂ SO₄ +85.2 milliliters HNO₃ for10 seconds at 35° C. to produce material having a front to backresistance of about 0.23 milliohms. After being rinsed with distilledwater and dried, an area of about one square inch of a predesignatedmating surface of one copper coupon and one nickel coupon was evenlycoated with a thin layer of conductive coating 25 having a compositionof about 23% indium and 77% gallium, using a cotton swab applicatorafter which said coated surfaces were pressed together to form aconductive copper-nickel couple. This was placed in an oven set for anominal temperature of about 90° C. For purposes of comparison a cleanedbut uncoated copper-nickel couple made from the remaining coupons wasalso placed in the oven. Both couples were also loaded to 10 psi tosimulate both thermal and mechanical levels experienced in a typicalchlor-alkali cell electrode installation. When assembled, no bonding wasexperienced with either couple.

The resistance across the couples was periodically measured with resultsas follows:

    ______________________________________                                                    [Cu--Ni Couple Resistance]                                                    (Milliohms)                                                       Aging Time    Coated     Uncoated                                             ______________________________________                                        0 days                   0.55                                                 1 day.sup.    0.21                                                            3 days        0.19                                                            4 days                   2.80                                                 6 days                   63.50                                                8 days        0.21                                                            10 days       0.07                                                            11 days                  142.5                                                15 days       0.19                                                            17 days                  135.6                                                25 days       0.14                                                            ______________________________________                                    

The results of this example show that whereas the contact resistance ofthe untreated couple increased rapidly, that of the .[.coupled.]..Iadd.couple .Iaddend.remained stable and may have actually decreasedslightly after 25 days of testing.

EXAMPLE 2

The procedure of Example 1 was repeated with the nickel coupon beingreplaced with 0.00385"×1"×2" titanium (Grade 1) coupons having, afteretching, a front to back resistance of about 1.75 milliohms.

Results obtained are given below:

    ______________________________________                                                    [Cu--Ni Couple Resistance]                                                    (Millohms)                                                        Aging Time    Coated     Uncoated                                             ______________________________________                                        4 days        2.60       170                                                  8 days        0.95       190                                                  ______________________________________                                    

Results comparable to Example 1 were observed. Note, however, howrapidly and to what degree the electroresistant oxide coating builds upon the titanium surface.

EXAMPLE 3

The procedure of Example 2 was followed with the titanium being replacedby titanium coupons containing a 0.1 micron thick layer of coppersputtered onto the mating surface. This was cleaned using the procedurefor copper as detailed in Example 1.

    ______________________________________                                                 [Cu--Cu Sputtered Ti Couple Resistance]                                       (Milliohms)                                                          Aging Time Coated        Uncoated                                             ______________________________________                                        0 days                   0.4                                                  1 day.sup. 0.98          4.4                                                  3 days     1.03                                                               4 days     0.63          50.5                                                 6 days                   60.5                                                 8 days     0.63                                                               10 days    0.64                                                               11 days                  70                                                   15 days    0.65                                                               17 days                  70                                                   25 days    0.85                                                               ______________________________________                                    

This shows that sputtering copper on titanium produces a system whichwhile superior to a Cu-Ti couple will still quickly break down onlong-term exposure to the temperature and pressure environmentalconditions of a cell to achieve uncoated contact resistance valuessubstantially higher than those found with coated couples.

This invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. An electrode for use in an electrolytic cell,said electrode being comprised of an internal copper conductor and anexternal element, said element being selected from the group consistingof titanium and nickel, at least a portion of said copper conductorhaving contact surfaces which are held in intimate contact with contactsurfaces of said external element, said conductor and said elementhaving at an area of contact a conductive coating between said contactsurfaces of said conductor and said element, said conductive coatingbeing comprised of a mixture of between about 20 and about 30 percentindium by weight and about 80 and about 70 percent gallium by weight,whereby the contact resistance between said conductor and said elementis reduced.
 2. The electrode of claim 1 wherein said conductive compoundis comprised of between about 23 and about 26 percent by weight ofindium and about 77 and about 74 percent by weight of gallium.
 3. Anelectrolytic cell containing the electrode of claim
 1. 4. The electrodeof claim 1 wherein the titanium further has a layer of copper on saidcontact surface. .Iadd.
 5. An electrode for use in an electrolytic cellsaid electrode comprising in combination:(a) copper means having atleast a first mating contact surface for conducting electrical energy;(b) electrolyte corrosion-resistant means having a second mating contactsurface which contacts the conductor means along the first matingsurface to prevent corrosion of the conductor means, thecorrosion-resistant means being selected from the group consisting oftitanium and nickel; (c) electrode surface means connected to theconductor means; and (d) an electrically conductive coating in contactwith the at least first mating contact surface of the copper conductormeans and the second mating contact surface of the corrosion-resistantmeans, the electrically conductive coating being a liquid-metal mixturecomprising between about 20 and about 30 percent indium by weight andbetween about 80 and 70 percent gallium by weight such that theelectrical contact resistance between the conductor means and thecorrosion-resistant means is reduced. .Iaddend. .Iadd.6. The electrodeaccording to claim 5 wherein said conductive compound is comprised ofbetween about 23 and about 26 percent by weight of indium and about 77and about 74 percent by weight of gallium. .Iaddend. .Iadd.7. Theelectrode according to claim 5 wherein the corrosion-resistant means ismade of titanium and has a layer of copper on the mating contactsurface. .Iaddend. .Iadd.8. An electrode for use in an electrolytic cellhaving copper conductor means with at least a first mating contactsurface for conducting electrical energy, electrolytecorrosion-resistant means having a second mating contact surface whichcontacts the conductor means along the first mating contact surface, thecorrosion-resistant means being selected from the group consisting oftitanium and nickel, and electrode surface means connected to theconductor means, the improvement comprising: an electrically conductivecoating in contact with the at least first mating contact surface of thecopper means and the second mating contact surface of thecorrosion-resistant means, the electrically conductive coating being aliquid-metal mixture comprising between about 20 and about 30 percentindium by weight and between about 80 and about 70 percent gallium byweight such that the electrical contact resistance between the conductormeans and the corrosion-resistant means is reduced. .Iaddend. .Iadd.9.The electrode according to claim 8 wherein said conductive compound iscomprised of between about 23 and about 26 percent by weight of indiumand about 77 and about 74 percent by weight of gallium. .Iaddend..Iadd.10. The electrode according to claim 8 wherein thecorrosion-resistant means is made of titanium and has a layer of copperon the mating contact surface. .Iaddend.